A portion of the disclosure of this patent document may contain material which is subject to copyright protection. To the extent that it does, the copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. The following notice applies to the software and data as described below and in the drawings hereto: Copyright (copyright) 2002, Mentor Graphics Corp., All Rights Reserved.
This invention relates to the field of printed circuit board design, and more specifically, to a mechanism for automatically and intelligently creating milling paths for breaking out boards from a panel.
A PCB starts out as a schematic in which the PCB functionality is laid out as a logical diagram of symbolic representations. When the schematic is finished, the schematic is interpreted, or captured, into a PCB design of physical components that make up the PCB functionality as detailed in the schematics. The PCB design may then be manufactured into an electrical circuit.
For economic reasons, many PCB designs are typically placed on a single panel during the manufacturing process, and each design is then populated with the components to form an electrical circuit. Once the populated PCBs have been tested, they must be broken out from the panel. Under the existing state of the art, there are three processes for creating those board breakout patterns from the panel: scoring, shearing, and milling.
Shearing is the process where a panel is placed into a shearing cutter (guided by a pair of guide pins on each side), and the board is sheared cleanly. Shearing is used to create rectangular profile boards, and is normally done by performing all cuts of the longest sides of the panel, and then the remaining shorter sides.
Disadvantages of the shearing process include: 1) the cost of the shearing machine; 2) additional workstation setup for a factory floor; 3) high probability of solder stress fractures to the solder joints caused by excessive shearing force, which can result in soldered parts breaking off the board; and 4) on thicker boards, the potential exists for delaminating the traces and copper areas caused by stress induced shearing.
Scoring is a circuit board fabrication method used to make long, straight cuts along the entire length and/or width of a panel, and therefore is often used to create rectangular profile board shapes. Scoring is the process where a xe2x80x9cVxe2x80x9d groove (usually no more than 0.005xe2x80x3 wide on the surface of the panel) is cut ⅓rd  into the top of the panel along an entire X or Y coordinate, and a xe2x80x9cVxe2x80x9d groove is cut ⅓rd  into the bottom of the panel along the identical X or Y coordinate. What is left is the middle ⅓rd  of the panel itself, and this is normally thick enough to endure the rigors of assembly and soldering, but easy enough to break by placing the panel into a single grooved fixture that will allow an operator to snap the unused panel material away from the finished boards.
Disadvantages of scoring include: 1) most PCB fabricators do not perform this process within their factories, which adds to the cost of manufacturing the PCB, as well as delays in delivery, for example; 2) it is very difficult to maintain a consistent depth of scoring if the panel has experienced any warpage during fabrication/shipping.
Another disadvantage of the scoring and shearing processes is that they are designed for breaking out square or rectangular PCBs, and therefore, do not properly address the need for PCBs to custom fit new enclosures. Another breakout process, called milling, is able to address this need. Milling requires the creation of a milling path around the entire PCB outline, interrupted by support tabs placed at predetermined distances around the board outline. A router with a single diameter drill bit is then used to cut out the board along the milling path.
Support tabs, which can be created with a combination of milling and drilling, help the panel remain rigid throughout all assembly and soldering processes. Support tabs are designed to be easily cut open by either pneumatic or hand-held cutters after the assembly and soldering of the panel has been completed. Historically, within CAD (Computer Aided Design) tools, support tabs have been created uniquely (i.e., one by one), or have been saved as a pre-route pattern template that can be placed repeatedly along a board outline.
One of the problems with using milling in this manner is being able to place support tabs where they do not cause assembly problems, or problems further in the process. For example, the placement of a protruding support tab underneath a protruding component may seem like a logical place for placing the support tab. However, the fabricator may not recognize that the protruding component must fit in a card cage, and a protruding support tab would prevent such functionality.
More recently, panels have been designed with internal support tabs, i.e., support tabs that are located within the board outline. One of the advantages of this process is that the entire edge feature is true to the board outline once it is broken off from the panel. Some of the disadvantages are that 1) the internal support tabs use valuable PCB design area; and 2) the pre-routes are typically not attempted until the PCB design is completed.
In one aspect of the invention is a method for intelligently and automatically creating milling paths. The method includes creating keepout regions on a PCB design having a PCB outline and critical components placed thereon, the keepout regions based on the outline and critical components; calculating support tab placements in accordance with the keepout regions; and placing support tabs on the PCB design in accordance with the calculated support tab placements.